I finally found the feature I was looking for. In some older versions of Firefox, you could type “dict word” into the address bar and it would return the Dictionary.com definition of the word. At some point, this built-in special search was taken out, and replaced with the default search from the address bar, Google’s “I feel lucky”.

This article details how to add in your own custom search keywords to search just about any site. All you need to do is make a bookmark for the search string, e.g. http://dictionary.reference.com/search?q=%s, where %s is a placeholder for the search string; Go to Manage Bookmarks; Click Properties for the bookmark, and type a keyword in.

.

I am elated.